The acquisition and saving of a large number of possessions that interfere with the use of living areas in the home are remarkably common behaviors that can pose serious threats to the health and safety of the affected person and those living nearby. Recent research on hoarding has led the DSM-5 Anxiety, Obsessive-Compulsive Spectrum, Post-traumatic, and Dissociative Disorders Work Group to propose the addition of hoarding disorder to the list of disorders in the upcoming revision of the diagnostic manual. This review examines the research related to the diagnosis and assessment of hoarding and hoarding disorder. The proposed criteria appear to accurately define the disorder, and preliminary studies suggest they are reliable. Recent assessment strategies for hoarding have improved our understanding of the nature of this behavior. Areas in need of further research have been highlighted.